Looking to build a Ma7da Locost racer targeting the start of next season but having never welded before thinking a pre-made chasis might be a good idea, is there anywhere that sells chasis suitable for this? or failing that would it be a horrible idea to weld one up myself?

Anybody can learn to mig weld to a standard suitable for joining new 25mm square tube together for a chassis in less than a day. When you have finished the chassis you will still know how to weld (which is not always a blessing as you keep being asked to fix other peoples suff.....). Do not buy the cheapest welder you see.......

The first question you need to answer for yourself is. Are you looking at being competitive or just wanting to drive round as there is a huge difference in how you will have to approach it.
And dont think it is LOCOST for one moment, we converted a locost to sports specials specification that won class B and 2nd in the overall championship cheaper than it was to convert to the Ma7da championship.

MycoLogic as your new here PROCOMP knows what he's on about. He's a go to person for setup / running a racecar (or setting up a road car).

Would definitely be worth having a proper chat with him & joining the locost racers 750mc group on Facebook